Firefox 17 will require OS X 10.6 and later, so you need to update your Mac if you want to continue using Firefox beyond Firefox 16.
You can see the Firefox 17 system requirements here:
*http://www.mozilla.org/en-US/firefox/17.0/system-requirements/
*http://blog.mozilla.org/futurereleases/2012/10/04/we-bid-you-adieu-spotted-cat/